Simplify return statements.

Remove redundant ‘this’.
Remove unused using directives.
Simplify LINQ chains.
Add some trait property descriptions.
Add readonly where viable.
Add fullstops to some yaml descriptions.

OpenRA.Mods.D2k/Traits/Render/WithAttackOverlay.cs

@@ -10,8 +10,6 @@
 
 using OpenRA.Graphics;
 using OpenRA.Mods.Common.Traits;
-using OpenRA.Mods.RA;
-using OpenRA.Mods.RA.Traits;
 using OpenRA.Traits;
 
 namespace OpenRA.Mods.D2k.Traits